The present invention relates generally to disposable absorbent bandages, particularly to disposable diapers, and, more particularly, to a new and improved disposable absorbent diaper pad construction exhibiting low wet density and improved pad integrity in use.
In recent years, improvements in disposable diapers have revolutionized the diapering of infants. As the term "disposable" implies, these diapers are designed to be discarded after single use. Disposable diapers generally consist of an absorbent pad, a pad-covering topsheet which contacts the infant, and a liquid-impervious backsheet for containing liquid wastes within the absorbent pad. U.S. Pat. No. Re. 26,151 which issued to Duncan et al. on Jan. 31, 1967 and which is hereby incorporated herein by reference is representative of such prior art disposable diaper structures.
Various materials have been employed as the absorbent media in such disposable absorbent structures. When plies of creped cellulose wadding are employed as the absorbent media there is little or no need for enclosing the absorbent media within an envelope of wet strength tissue, since the creped cellulose wadding generally has sufficient strength to provide satisfactory in use pad integrity. When a low-strength material such as airlaid wood pulp fluff, commonly referred to as airfelt, is employed as the absorbent media, a strength-imparting envelope is generally necessary not only to provide satisfactory in use pad integrity, but also to avoid the dusting and linting problems commonly associated with such low-strength absorbent materials both during manufacture and in use. Failure to provide sufficient in use pad integrity in a disposable diaper adversely affects both its absorbency and its containment characteristics.
For absorbent pads comprised of conventional chemically processed wood pulp fibers which have at least some degree of cohesive strength in airlaid form it has been the practice to employ a pair of wet strength tissue plies to encapsulate the absorbent media to impart at least an adequate degree of in use pad integrity. U.S. Pat. No. 3,952,745 issued to Duncan on Apr. 27, 1976, said patent being hereby incorporated herein by reference, is representative of such structures. The lack of pad integrity in the absorbent fibrous media reaches critical proportions, however, when relatively stiff high yield wood pulp fibers, such as thermomechanical wood pulp fibers, which have very little cohesive strength in airlaid form are utilized as the absorbent medium. As utilized herein, high yield wood pulp fibers are considered to be those wherein the bulk of the lignin is not removed during processing. Thus, a much greater proportion of the tree from which the fibers are obtained is converted into fibers.
By way of contrast, conventional chemically processed wood pulp fibers are considered to be those wherein the lignin is substantially removed during processing. High yield fibers are typically non-collapsed, stiffer, and more resilient than the collapsed ribbon-like chemically processed fibers. Accordingly, there is less interentanglement of the high yield fibers with one another and consequently less cohesive strength in airfelts produced therefrom. Nonetheless, it is extremely desirable to utilize such fibers in disposable absorbent products, not only from the standpoint of more efficient utilization of natural resources due to reduced waste in converting the fibers, but also from the standpoint of reduced fiber utilization for an equivalent absorptive capacity. The relatively stiff and springy nature of high yield fibers provides an airfelt pad which exhibits a lower wet density in use than a pad formed from conventional chemically processed wood pulp fibers. Consequently, for comparable quantities of fibers, high yield wood pulp fiber pads exhibit greater void volume in use, and consequently greater absorptive capacity.
To date, the difficulty experienced with such pads, even when encapsulated in an envelope of wet strength tissue paper, has been that the greater void volume could not be effectively utilized to absorb and retain body exudates due to a lack of in use pad integrity. The actions of the wearer typically cause such pads to disintegrate, thereby producing a wet strength tissue envelope containing a mass of loose, unbonded fibers which are free to shift about the interior of the envelope. Such structures exhibit extremely low void volumes in use and are clearly ineffectual in the absorption and retention of body exudates.
Various means to strengthen or reinforce airfelt pad structures are well known in the art. Typical approaches to this problem are the addition of adhesive coated scrims internal to the pad and the application of reinforcing resins or foam forming agents to the exposed fibrous surfaces of the pad. However, such prior art solutions to the problem of in use pad integrity in absorbent fibrous structures are relatively high in cost, thereby adversely affecting the overall economy of use of such disposable absorbent structures. Furthermore, such solutions are often too complex to reliably execute, and in general cannot readily be carried out without major design changes in the disposable absorbent structure per se.